Increases in semiconductor chip input/output (I/O) in high-performance computers and other systems, and the shrinking size of consumer and automotive electronics are driving increased wiring density in single and multi-chip packages and printed wiring boards. See Microelectronics Packaging Handbook, 2nd edition, pp. 11-63, 11-64, and 11-117 (1997) by Tummala, Rymaszewski, and Klopfenstein. To achieve increased wiring density, the signal traces on these chip packages and printed wiring boards, and the spaces between these traces, are becoming narrower. As wiring density increases, limitations in the photolithographic processes used to define the signal traces have become apparent. These limitations manifest themselves in a decreased yield of chip packages or printed wiring boards due to defects in the signal traces.
For example, one common process for defining signal traces on chip packages, printed wiring boards, or other electronic interconnects is known as a subtractive process. In this process, a panel consisting of a copper clad dielectric material is coated with a photoimagable polymer (photoresist). A phototool containing a negative image (clear traces with a black background) of the signal trace layer to be formed is placed over the photoresist coated panel and ultraviolet (UV) light is used to expose and crosslink the photoresist through the clear areas in the phototool, which correspond to the subtractive signal image. Unexposed areas of the photoresist are then removed in a developer solution. The traces are then defined in the copper by exposing the panel to a copper etching solution, where the photoresist acts to protect the copper beneath it, while the remaining copper is removed. The photoresist is then removed to complete the process.
As signal trace widths become narrower, defects may occur in the signal traces due to a variety of factors. The presence of dirt or debris on or beneath the phototool during exposure can lead to breaks or opens in the signal traces. Poor adhesion of the photoresist to the copper, or mechanical damage to the photoresist during processing can result in similar trace opens. Both adhesion related problems and mechanical damage become more prevalent as trace widths are narrowed. Most of the defects caused by debris during the exposure process, poor photoresist adhesion, or mechanical damage have been found to occur randomly across the surface of a panel.